PROJECT SUMMARY/ABSTRACT The successful treatment of conditions such as cancer, metabolic disorders, and infectious diseases has been enabled by a detailed understanding of the structures of the disease-causing agents, permitting design of effective chemical interventions. In contrast, a lack of structural information about disease targets has impeded the development of therapies for systemic amyloid diseases. The targets in these diseases are protein aggregates called amyloid fibrils; they accumulate in body organs and lead to eventual organ failure. Amyloid fibrils are difficult to study by traditional structure-determination methods, but our recent efforts have yielded significant results. We have identified and determined the amyloid structures of the aggregation-driving segments from two disease-causing proteins: transthyretin, which causes transthyretin amyloidosis (ATTR), and immunoglobulin light chain, which causes light-chain amyloidosis (AL). Our transthyretin amyloid structures have enabled us to design effective inhibitors of transthyretin aggregation; they cap the ends of the developing amyloid fibrils, preventing their growth. In the new grant period, we aim to (1) further our understanding of amyloid structure, and (2) apply this understanding to the development of new and better candidate therapeutics and diagnostics. In Aim 1, we propose to determine near-atomic resolution structures of ATTR and AL patient-derived amyloid fibrils, using cryo-electron microscopy. We will use these structures to design and further optimize amyloid- capping inhibitors for both transthyretin and immunoglobulin light chains, testing their effectiveness on isolated proteins and fibrils. In Aim 3, we will evaluate the transthyretin inhibitors in worm and mouse models of ATTR, as the next steps in moving these potential therapies toward use in humans (clinical trials). In Aim 2, we address the present difficulty of diagnosing and monitoring systemic amyloid diseases. We take advantage of the specific binding of our fibril-capping inhibitors to amyloid fibrils to develop a safe, non- radioactive diagnostic for ATTR. Our inhibitors will be coupled to injectable magnetic nanoparticles that are detectable by magnetic resonance imaging (MRI). Thus, once injected into patients, the inhibitors will bind the nanoparticles to amyloid fibrils in organs, permitting detection of pathogenic aggregates by MRI. In Aim 4, we will capitalize on the natural protective role of transthyretin against fibril formation and toxicity of ?-amyloid, linked to Alzheimer's disease. We have identified the minimum segment of transthyretin that confers such protection and we now aim to assess its potential as a treatment of Alzheimer's in a mouse model. Our discovery of effective chemical interventions for amyloid diseases could prevent suffering and extend the lives of millions of Americans afflicted with these fatal conditions. Moreover, our proposed diagnostic tool will enable doctors and researchers to evaluate the usefulness of emerging therapies. The same methods we propose here may be eventually used to develop inhibitors and diagnostics for other amyloid diseases.